Known electrothermal ink print heads--i.e. using the so-called bubble-jet principle--are typically provided with a plurality of individual jets or nozzles from which individual droplets of a defined or predetermined or controllable magnitude are generated under the command of an electronic control unit and are discharged or propelled or ejected or shot outward in an intended pattern in the direction of an image-receiving print medium or substrate or the like.
The lines of image or test characters to be printed are produced by a plurality of ink droplets that are arranged in rows and columns in the general form of a matrix.
In an advantageous manner, each of the columns of such a character-oriented matrix is printed serially one after another so as to provide and enable high printing speed and uniform type faces.
Thus, an ink print head suitable for the aforementioned printing process must combine a plurality of substantially identical elements capable of shooting out or ejecting ink droplets at selectively-controlled times, commonly known as "drop-on-demand" discharge. A characteristic feature of this technology conventionally consists in the provision of an electrically-connected resistor or the like forming a heating element and located in a capillary tube that is filled with recording liquid, as for example printing ink, in the vicinity of the capillary tube opening. When a predetermined thermal or heat energy is selectively supplied to this heating element by the application of a brief current pulse through the resistor, a rapidly expanding ink steam bubble is initially produced through an extremely rapid transfer of heat to the ink liquid. The bubble bursts relatively quickly after the supply of energy to the resistor terminates and the liquid ink cools. The pressure wave formed by the steam bubble in the interior of the capillary tube permits an ink drop of limited mass to propellingly exit from the nozzle opening onto the surface of a print medium or substrate located in appropriate proximity to the opening.
One advantage of the bubble-jet principle is that the relatively large and rapid change in volume necessary for propellingly discharging the ink droplets is obtained from a very small active converter surface area by utilizing the alternating liquid-gas-liquid phases of the liquid ink. The small converter surfaces, in turn, allow the ink print heads to be constructed in a relatively simple and inexpensive manner characterized by high printing track density and small dimensions using modern production methods such as high-precision photolithography processes in lamination or film technology.
An ink print head including a chip and an associated ink supply vessel is disclosed in International Patent Application PCT/DE 91/00364, in which construction the chip is mechanically locked onto or coupled to the ink supply vessel by mounting or assembly clamps. The chip includes ink ducts that are closed on three sides and open on the fourth and which are separated one from another by thin, substantially trapezoidal intermediate duct-walls. The closure of each respective ink duct in the ink shooting or discharge direction is formed by a thin diaphragm which, in turn, includes the discharge nozzle of the respective ink duct.
A surface of the ink supply vessel forms the external closure of the ink ducts along the fourth side which is open toward the chip. As the chip is held on the ink supply vessel by only the positively-locking assembly clamp, the intermediate duct-walls are movable on and relative to the surface of the ink supply vessel. This disadvantage is aggravated by a high length-to-thickness ratio of the intermediate duct-walls on the order of approximately 50:1 and a high height-to-thickness ratio of those duct-walls of approximately 10:1.
When an individual heating element of the print head is triggered for producing and discharging an ink droplet, the heating element, and the formation of a bubble, results in the creation of a localized overpressure in the respective ink duct. This overpressure condition leads not only to the intended ejection or discharge of the ink droplet but, in addition, to a bending or deformation or distortion of the intermediate duct-walls bounding the respective ink duct in the direction of the immediately-adjacent ink ducts. The effect is that, in addition to the amount of energy required for ejecting the droplet, there must be a further application of that so-called lost energy which results in deformation of the intermediate duct-walls. The need to supply this lost energy in the printing process necessarily reduces the overall operating efficiency of the ink print head.
Moreover, the deformation or bending of the intermediate duct-walls seriously affects the operation and status of immediately adjacent ink ducts. That is, when triggering an ink duct adjacent to an ink duct which is not being triggered, the latter may nevertheless unintendedly discharge a droplet as a result of the increased pressure in the intentionally-triggered duct.
Depending upon whether adjacent ink ducts are triggered, there is a change in the pressure ratios in the respective ink ducts and, accordingly, in the resulting mass of the drop and the print quality.